The pivot-shift phenomenon: a clinical and biomechanical perspective

The Knee 5 Ž1998. 141]158

Review article

The pivot-shift phenomenon: a clinical and biomechanical perspective Anthony M.J. BullU , Andrew A. Amis Biomechanics Section, Mechanical Engineering Department, Imperial College of Science, Technology and Medicine, Exhibition Road, London SW7 2BX, UK Accepted 12 October 1997

Abstract The current literature on the assessment, treatment and effect of the pivot shift is reviewed. Various questions pertaining to the cause of the pivot shift, the three-dimensional dynamic kinematics and the specific procedures required to reconstruct a knee that demonstrates the pivot shift are found to be unanswered. Biomechanical studies are presented, but it is unlikely that any research will be able to refute or reconcile all of the conflicting statements published in the literature, because of the notable differences in geometry, anatomy and soft tissue properties between different knees. However, there is scope for clarifying and explaining these results. Q 1998 Elsevier Science B.V. All rights reserved. Keywords: Pivot shift; 3-Dimensional dynamic kinematics; Knee reconstruction; Biomechanical studies

1. Introduction A number of clinical examinations have been devised to test for the presence of instabilities. These are used in the diagnosis of ligament injuries and also in the assessment of the post-operative results of ligament repair and reconstruction. These procedures assess the ‘static’ instabilities and the dynamic instabilities. ‘Static’ clinical tests are a measurement of abnormally increased joint laxity due to soft-tissue injury. These uni-axial tests for soft-tissue injury to the knee are based on applying a load or a displacement to the joint and measuring the resultant displacement or restraining force, respectively. A simple example of a ‘static’ instability test for a ruptured ligament would

U

Corresponding author. Tel.: q44 171 5895111 ext. 57101; fax: q44 171 8238845; e-mail: [email protected] 0968-0160r98r$19.00 Q 1998 Elsevier Science B.V. All rights reserved. PII S0968-0160Ž97.10027-8

be the application of a load or displacement in the direction of primary restraint of that ligament. In Fig. 1 a schematic model of a planar joint is presented. Two bones are connected by two ligaments, which can be modelled as springs. One of these ligaments is ruptured } the other is intact. The clinical test displaces Bone One in a direction that stretches the ligament under test, while ideally not stretching the other ligaments. Therefore a clinical test for rupture of Ligament Two would consist of applying an anterior displacement or load to Bone One. Resolving the forces on Bone One in the anterior direction shows a very small contribution to the force from Ligament One. This force is minimised by applying the load in the anterior direction. Likewise, a load applied to Bone One causing it to displace in a postero]distal direction would test for a rupture of Ligament One, whilst minimising the restraint due to Ligament Two. Therefore the principle of these ‘static’ uni-axial tests is to find the joint position and bone displacement

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Fig. 1. Schematic model of a joint with two ligaments.

that ‘isolate’ a particular ligament, thus allowing the tester to identify which individual structures are ruptured. A difficulty with these tests is that there is always more than one structure producing a restraining force when a displacement is applied. This introduces the concept of ‘primary restraints’ and ‘secondary restraints’, where primary restraints are the structures which provide most of the restraining force to a specific displacement and the secondary structures are those which, combined, provide only a small amount of the restraining force w1x. In general, the fibres of primary restraints are aligned with the displacements } so they are stretched, while secondary restraints are perpendicular to it and are not stretched. The results of clinical testing for a degree of injury are usually graded. For example, Grade 0 is considered as normal; Grade 1 Žmild. is a displacement of less than 5 mm; Grade 2 Žmoderate. is a 5]10 mm displacement; Grade 3 Žsevere. is a displacement of 10]15 mm; and Grade 4 indicates displacement of ) 15 mm w2x. Correlating these ‘grades’ to ligament ruptures is difficult. Some knees may have a mild displacement, but also may have a complete rupture of the ligament which is a primary restraint to the displacement applied. This can be explained by the strong and stiff secondary structures which can be more significant in some joints than in others. Another approach which can be used in conjunc-

tion with the ‘Grade’ description for degree of injury is categorising the end point of displacement in a clinical test. A soft or ‘mushy’ end-point signifies total rupture of the ligament being assessed. This correlates to Fig. 1, where an increased displacement in the direction of the applied force results in a very small extension of Ligament One, the secondary restraint. Therefore the restraining force will be increased by a small amount for the displacement applied. Thus the joint stiffness will be low which correlates with the ‘mushy’ end-point. A hard end-point signifies that some of the primary restraint is intact w3x. This can also be explained in terms of Fig. 1. If part of Ligament Two is intact then the stiffness of the joint will be high due to the intact portion of the ligament being aligned in the direction of the applied displacement or load. This high joint stiffness correlates with the hard end-point. Dynamic instabilities are commonly presented to the clinician as symptoms, such as ‘giving way’ or ‘buckling’ during daily, or athletic, activity. The dynamic tests mimic these symptoms in the clinical setting by controlled loading of the joint with movement. All these tests require great skill and subtle application since they can induce painful sudden motions, against which the patient guards by tensing his muscles, and also because the motions are complex and difficult to analyse. Two main problems are identified. Firstly, the motions induced by the examiner

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are felt and described in a subjective manner thus proving difficult to quantify or grade. Secondly, the loads and displacements applied by the examiner are not constant or easy to measure. Therefore the mechanisms of these dynamic instabilities are not strictly defined or even known. The most widely recognised dynamic instability is the phenomenon known as the pivot shift. Several terms have been used to describe this phenomenon: ‘pivot shift’ w4x, ‘antero]lateral rotatory instability’ w5x and ‘jerk sign’ w6x. In this review, the term ‘pivot shift’ will be used for all indications of this rotatory instability. 2. The pivot shift The pivot shift is a subjective symptom that is experienced during athletic, or even normal, activity after ACL rupture and a physical sign that can be demonstrated on the examining table. The pivot shift is a complex rotational and translational instability of the tibio]femoral joint. It has been shown that the presence of the pivot shift correlates with reduced sports activity w7x. Most investigations of the pivot-shift phenomenon show that a complete or partial tear of the ACL is required for the clinical sign to be present w8x. Several studies have investigated the pivot shift, both clinical and experimental. The clinical studies have tended to focus on the treatment of the pivot shift of the knee based on clinical data. Experimental studies investigating the cause of the pivot shift have been carried out in vitro. These point to both the effectiveness of clinical tests in assessing the pivot shift, but also the subjectivity of the tests. There are a number of important contributors to the pivot shift } in diagnosis and intensity. These are: the ACL integrity, the dynamic loading of the popliteus muscle, the iliotibial tract, the geometry of the femoral condyles and tibial plateau, the valgus moment applied, the rotational axis of the tibia as well as the contribution due to all the other soft-tissue structures of the knee. 3. History The term ‘pivot shift’ was coined in 1972 by Galway et al. w4x. This, however, is not the earliest documentation of this instability. In 1914 Jones and Smith w9x described a ‘slipping knee’ in a case report. ‘On placing the hands on the joint the femur seemed to be suddenly displaced inwards just before extension was completed and this constituted the ‘slipping’ of which the patient complained’. Smith also presented the following symptoms for knees with ACL and medial meniscus injury in 1918 which included the phrase ‘rocking or slipping of joint, combined with a feeling

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of insecurity’, and, ‘external rotation of the tibia was increased’ w10x. In 1920 Hey Groves commented on the symptoms of ACL injury involving a ‘jerk’ as the ‘tibia slips forward on the femur’ w11x. However, no mention was made of rotatory instability. Palmer w12x described the symptomatic cruciate deficient knee as ‘giving way’ and ‘crumples up’ } typical descriptions of the pivot shift. These phrases were also used by Abbott et al. w13x who said that ‘in a rupture of either cruciate ligament, the patient complains that the leg ‘crumples up and gives way’ and that ‘there is a feeling of complete insecurity when the knee moves into a position of flexion’. Attention was drawn to the rotatory component of instability when Slocum and Larson w14x Žquoted in w15x. recognised a rotatory instability as a functional deficit in the running weight-bearing leg. This history shows that examiners have observed the pivot shift of the knee joint and associated it with some form of soft-tissue injury since the beginning of this century, however, there was little clarity as to the specific cause. For example, Abbot et al. w13x suggested that an injury to either the PCL or the ACL would produce the described ‘giving way’ symptom. 4. Clinical description Several clinical tests to determine the presence of the pivot shift have been proposed. They can be classified broadly into two main types: the reduction test and the subluxation test. In the reduction test, the knee is flexed from full extension under a valgus moment. A sudden reduction of the anteriorly subluxed lateral tibial plateau is seen as the pivot shift. This was first described by Galway et al. w4x as a dynamic instability occurring with an ACL injury. Slocum et al. w5x proposed a similar test called the antero]lateral rotatory instability test. In this test the patient lies on his sound side with the unstable knee up and flexed 108 and the medial aspect of the ipsilateral foot resting on the table. The patient maintains his ipsilateral pelvis rotated posteriorly 30]508. The knee is pushed into flexion and a sudden reduction of the anteriorly subluxed lateral tibial plateau is observed. This was devised specifically for the heavy or tense patient in whom the knee is difficult to manipulate. MacIntosh’s lateral pivot-shift test, as reviewed in the literature, was very similar to Galway’s test, in which the patient is supine and relaxed. The internally rotated foot is lifted off the examining table with the knee extended. The knee is flexed while applying valgus stress. Because of the similarity of MacIntosh’s and Galway’s tests and the fact that they co-presented the first work on the pivot shift, most clinicians attribute the pivotshift test to them both. Bach et al. w16x found that the

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most sensitive method of conducting the pivot-shift test was with the hip in abduction and the tibia in external rotation. Noyes et al. w17x suggested a modification of the pivot shift starting with the tibia in external rotation and using a technique that enhances anterior tibial translation and avoids excessive tibial rotation. It must be noted that this modification was proposed based on 11 surgeons assessing a single cadaveric knee. The subluxation test is effectively the reverse of the reduction test. The knee is extended from a flexed position under a valgus moment and internal rotation of the tibia. The sudden anterior subluxation of the lateral tibial plateau is observed as the pivot shift. This was introduced in 1976 by Hughston et al. w6x and was called the jerk test. They promoted this as being more sensitive than the reduction test of Galway et al. w4x. Losee et al. w18x proposed a modification of Hughston’s subluxation test. With the patient supine and relaxed the knee and hip are flexed to 458. The tibia is rotated externally and the knee is extended slowly while applying a valgus torque. Other tests which have been assessed to demonstrate the functional pivot-shift are discussed by Tibone et al. w19x, such as Arnold’s ‘crossover’ test, in which the patient steps with his sound leg across the involved limb. This demonstrates the antero]lateral instability, but with the patient in an erect, weightbearing position w20x. In this review the term pivot shift will be used for all indications of this instability. 5. Long-term effects of the pivot-shift instability Many authors agree that due to the detrimental long-term effects reconstruction is required for active patients who display the pivot-shift instability. For example, Shelbourne and Rowdon w21x argued that the elite athlete requires reconstruction of an ACL injury to stop the positive pivot shift and other clinical indicators so that degenerative changes are halted or slowed down. In 1980 Galway and MacIntosh w22x also discussed the long-term effects of the pivot shift which include meniscal tears and articular cartilage erosions. These changes are well noted in the literature and include degenerative changes of the cartilage w23,24x, re-injury, meniscal damage and joint arthritis w25x. Reuben et al. w26x, when measuring the relative motion of cadaveric femora relative to their fixed tibiae, found that the pivot shift produced a sudden directional change in the motion of both femoral condyles. This may, therefore, be responsible for the documented meniscal degeneration which accompanies chronic ACL deficiency. Other authors w27x have presented data suggesting that conservative treatment can be effective even for those patients who display the pivot-shift instability,

but the evidence of progressive degeneration noted above means that this view currently has little support. 6. Grading the pivot-shift sign Grading the pivot shift serves two main purposes. It is claimed that the test is subtle enough for examiners to assess for different injuries and discriminate between them. Also, the severity of the pivot shift is critical in deciding which type of treatment } including corrective surgery } should be pursued. As for ‘static’ instabilities, grades emphasise the magnitude and also the suddenness of the motions, from ‘slight gliding subluxation’ to ‘gross irreducible subluxation’. The IKDC knee ligament standard evaluation form grades the pivot shift as glide Žq., clunk Žqq . or gross Žqqq . w28x. In 1987 Jakob et al. w29x presented an alternative method of grading the pivot shift based on the position of the tibia. Their technique was based on that promoted by MacIntosh and led to three grades: a Grade I pivot shift occurs only when the tibia is held in internal rotation. The pivot shift is absent in neutral or external rotation. It is barely seen when the patient is awake, but is more obvious under general anaesthesia. A Grade II pivot shift is positive in the neutral position as well as in internal rotation of the tibia. A Grade III pivot shift is observed when the tibia is held in neutral or moderate external rotation. In internal rotation the shift is less obvious. Similarly Bach et al. w16x assessed the effect of limb position on the pivot shift. They measured the pivot shift in 20 patients under general anaesthesia with the leg in six positions Žinternal, neutral and external tibial rotation; abduction and adduction of the hip.. Using the grading 0 s absent, 0.5s trace, 1.0, 2.0, 3.0 for the pivot shift at each position they assessed the effectiveness of eliciting the pivot-shift sign. They found that the pivot shift was easiest to elicit with the hip in abduction and the tibia in external rotation. This has recently been confirmed by Petermann et al. w30x. This probably relaxes the iliotibial tract and is not what might be expected. This is discussed in Section 11. 7. Reproducibility of the pivot shift It is well documented that different examiners elicit the pivot-shift instability in different ways and that some patients display the pivot shift under one clinician’s examination, but not another’s w17,31x. This may be due to expertise or varied techniques. Other factors are involved in reproducing the pivot-shift instability in the clinical setting. Acute or chronic conditions are different in their assessment, as are the

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results of assessment when the patient is awake or under anaesthesia, due to muscle spasm prompted by pain andror apprehension. 7.1. Testing under anaesthesia Various researchers have assessed the differences in the in vivo condition between eliciting the pivot-shift instability under clinical conditions with the patient under general anaesthesia, epidural anaesthesia or when fully awake. Norwood et al. w32x found that the jerk test Žsubluxation pivot-shift. was positive for nine out of 36 knees when the patients were awake, however, under anaesthesia all knees had markedly positive jerk tests. Similar conclusions have been reached by many researchers w31,33]36x. They attributed this to the patients ‘guarding’ against pain. Comparisons between results presented in the literature should bear in mind the method of assessing for the pivot shift, because of the noted effect of anaesthesia. This makes comparisons difficult as in many cases the authors of articles do not describe the clinical situation fully. 7.2. Position of the affected limb It has already been noted that the position of the affected limb can affect the severity or even the presence of the pivot shift ŽSec. 6.. However, there is not full agreement as to the body positions required for the best and most reproducible assessment of the pivot shift. Daniel w37x found that the pivot-shift grade was not affected by the position of tibial rotation. Various examiners have promoted placing the tibia in external rotation w16,17,30x. However, Jakob et al. w29x found that only the most severe pivot shifts occurred with the tibia in external rotation. Similarly for the jerk test, some examiners promote placing the foot in external rotation w18x, whereas others place the foot in internal rotation w6x. This evidence suggests that the position of the tibial rotation may not be important, but it is possible that these preferences have arisen as a result of different forces and motions imposed by different examiners. 8. Possible cause of the pivot shift From the literature the pivot shift has a rotational component } the rotation of the tibia about its long axis } and a translational component } the anterior subluxation of the lateral tibial plateau followed by its sudden reduction under certain loading conditions. These components are obviously linked. From a simple mechanical understanding of this motion the pivot shift might be caused by an injury to a structure, or structures, which are the primary restraints to ante-

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rior displacement of the lateral tibial plateau and to internal rotation of the tibia. Terry et al. w38x suggested that the pivot shift is a ‘result of the combined influence of the ACL, the mid third capsular ligament, the lateral meniscus and its capsular attachments and the capsulo-osseous and deep layers of the iliotibial tract’. This illustrates the complexity involved in analysing the mechanics and cause of the pivot shift, particularly as the anatomy of the structures in and around the knee is complex. Most authors agree that an ACL injury is required to produce the pivot-shift phenomenon w39x. However, there is not full agreement on any further specific injuries required to cause the pivot shift. For example, Norwood et al. w32x stated ‘there was no correlation between the different ligament injuries and the severity of the clinical instability’. Terry and Hughston w40x conducted an extensive study to define the symptomatic ACL deficient knee. They found that 83% of the knees with acute pivot shift had ACL tears. It is interesting to note that there have been other articles with documented ACL deficiency with no pivot shift apparent: vice versa, some knees with intact ACLs have presented positive pivot shifts } Chick and Jackson w41x on meniscectomy found 30 athletes with non-functionalrabsent ACLs with no positive pivot shift or jerk test instability and Fetto and Marshall w42x listed some knees which had positive pivot shifts with intact ACLs. Therefore one may hypothesise that the presence or absence of the ACL has differing effects in different knees, because of the variation in individual mechanics. Or there may be many false negatives when the patients are tested when awake w41x. 8.1. Isolated ACL injury On isolated ACL sectioning in a cadaver study Matsumoto w43x reproduced the pivot shift in 35% of the knees tested. Fetto and Marshall w42x conducted whole cadaver experiments on 50 knees. They sequentially sectioned or removed the ACL, ITT, LCL, popliteus tendon, biceps tendon and the lateral meniscus. They conducted clinical anterior drawer and pivot-shift tests on the knees without measuring the loading applied. Eighty-nine percent of their knees produced the pivot shift on isolated sectioning of the ACL. One of their knees Ž1r50. displayed the pivot shift on isolated sectioning of the ITT. These numbers are confirmed by various clinical articles discussing treatment of ACL injuries w44]50x. Others have shown that isolated ACL injuries will produce the pivot shift in all knees w35,51]59x. Losee et al. w18x found that no definite lateral capsular tears were visualised in 50 patients displaying the pivot shift. They did point out that stretching

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‘comparable to that seen in recurrent dislocation of the shoulder’ was seen. This lateral structure attenuation may be as a result of the pivot shift occurring due to an isolated ACL injury and, in time, stretching these structures. It may also arise as a result of the original trauma which resulted in the pivot shift. Although knees may be categorised as having ‘isolated’ ACL injuries, the magnitude of the abnormal bone to bone excursion which caused the ACL rupture is likely not to have left all other structures unaffected or unstretched } it is just that this is not easily recognisable post-injury. This may explain why laboratory investigations have a lower rate of pivotshifting, since they usually have a neat ACL cut with all other structures unstretched. Lintner et al. w60x found in vitro that partial rupture of the ACL Žspecifically the antero]medial band. did not produce a pivot shift. 8.2. ACL and lateral structures injury Losee et al. w18x found that all knees presenting a pivot shift had torn ACLs, however, they stated that tears of the lateral capsule could not be defined, but ‘it was observed to stretch sufficiently to allow the lateral tibial plateau to sublux anteriorly’. Later Losee w61x summarised that ‘the pivot shift is a symptom and sign of ACL and lateral and postero]lateral capsular deficiency of the knee’. Fifty-eight percent of the knees with a pivot shift examined by Norwood et al. w32x had both ACL and lateral capsular ligament injuries. The work of Fetto and Marshall w42x shows that all knees which had ACL injuries combined with lateral structure injuries displayed the pivot shift and Terry et al. w62x found that injuries to components of the iliotibial tract contributed to the variation in grade of the pivot shift. 8.3. Isolated lateral capsule injury Hughston et al. w6x found that a solitary lateral capsule injury could cause the pivot shift Žquoted by Matsumoto w43x.. This has been confirmed by Norwood et al. w32x who found that in a series of 36 knees which all displayed the pivot shift, six Ž17%. had lateral capsular ligament injuries with an intact ACL. Also a single knee with isolated ITT injury in Fetto and Marshall’s w42x experiments pivoted. 8.4. ACL and medial structures injury There is some agreement in the literature that the MCL must be intact for the pivot shift to occur. This is confirmed particularly as Matsumoto w63x found the rotational axis of the pivot-shift instability to be about the MCL: an injury to the MCL would take away that

pivot point. Lucie et al. w52x assessed 50 patients, two of whom had complete tears of ACL and MCL and did not display the pivot shift, whereas all those with isolated ACL injuries did display the pivot-shift instability. Gerber and Matter w33x had documented difficulty eliciting the pivot shift when, in addition to a rupture of the ACL, the MCL-complex was injured. In slight contradiction Donaldson et al. w35x found that all knees with isolated ACL ruptures displayed the pivot-shift instability, but only 89.5% when the ACL and MCL were injured. This would suggest that an MCL injury does allow the pivot shift to occur in some instances. However, the level of MCL injury was not discussed and we do not know, therefore, how much of the MCL complex was still intact and functioning. One could hypothesise that some portion of the MCL or postero]medial structures may be intact to have a tethering effect about which the pivot-shift motion could occur. Matsumoto w63x showed that when the ACL and MCL were divided the pivot shift did not occur. However, the method of reproducing the in vivo conditions in the laboratory on cadaveric limbs must be questioned in this study, because only seven of the 20 knees pivoted under his conditions when the ACL was sectioned. Also, Noyes et al. w17x cut the superficial fibres of the MCL as well as the ACL when conducting a cadaveric study on the pivot shift. This extra lesion might explain why some of the experienced examiners had difficulties eliciting the pivot shift in these experiments. 8.5. Summary of the causes of the pi¨ ot shift There seems to be general agreement in the literature that an ACL injury is a necessary condition for the pivot shift to occur, but also a solitary ACL injury does not always produce a pivot shift. The most common opinion in the literature is that, although some injury of the lateral structures is not a pre-requisite for the pivot shift to occur, those knees with combined ACL and lateral structure injuries Žor attenuation. are most likely to display the pivot shift. Some conflicting results are present, however, and these may be resolved by comparison of the loading applied to the joint, which is never presented in the clinical studies as the motions are ‘induced’ by the surgeon. The more valuable clinical studies are those in which the position of the joint is varied and the effect on the pivot shift is presented, for example, the articles by Jakob et al. w29x and Bach et al. w16x, Žcommented on in Section 6.. 9. Tibial rotation The pivot-shift motion has both rotary and transla-

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tion components. A measure of tibial rotation could shed some light on the cause, effect or removal of the pivot shift. Two approaches to measuring tibial rotation exist in the literature: 1. rotary laxity, which is the rotation of the tibia induced by a certain moment applied to it and which, therefore, has some constraint; and 2. coupled rotation, which is the change in the rotational position of the tibia with respect to the femur arising as a by-product of normal knee flexion]extension motion, in which no external restraints are applied. 9.1. Rotary laxity in ¨ itro Several investigators have studied rotary laxity of the tibia relative to the femur in vitro. Lipke et al. w39x measured the rotary laxity on cadaveric knees using a rig similar to the Oxford Rig for knees in extension to 408 flexion when loaded by a quadriceps tension of 224 N. Then the ACL, LCL and postero]lateral complex were dissected in different orders. They found that when the ACL was dissected Ž n s 7., internal rotation was significantly increased at 208 flexion with a joint load and that this was increased still further after the LCL and postero]lateral complex were dissected. When the LCL and postero]lateral complex were dissected first Ž n s 7., internal rotation was not increased, but this only increased significantly after the ACL was dissected. Therefore the quadriceps force pulls the lateral tibial plateau anteriorly near extension which is the first part of the pivot-shift motion prior to the sudden reduction of the internally rotated tibia. In the clinical situation this raises the question of the role of the quadriceps in inducing the pivot shift when the patient is standing on the affected leg. External rotation showed a statistically significant increase only after sectioning of the postero]lateral complex. Thus we can conclude from their work that the ACL is significant in restraining internal rotation of the tibia at low angles of flexion when the quadriceps are loaded. A similar study was conducted by Lane et al. w64x, but they found that ACL sectioning had no significant effect on internal or external tibial rotation with and without a joint load of 89 N. Gollehon et al. w65x also conducted a sequential cutting experiment and found that when the ACL, LCL and popliteus]arcuate complex were dissected together, a large increase in rotation was observed, but when these structures were dissected in isolation no significant increase in internal rotation was observed. Amis and Scammell w66x found very little difference

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in the rotational laxity of knees which had intra-articular reconstructions ŽIA. compared to those with IA combined with extra-articular reconstructions ŽEA.; post-reconstruction the knees were not significantly different to normal. The overall picture suggests that, while the isolated ACL has little role in controlling the overall range of tibial rotation, it does have a role in controlling the initiation of the pivot shift, when the joint is loaded. 9.2. Rotary laxity in ¨ i¨ o Zarins et al. w67x measured rotary laxity at various angles of flexion in vivo for 17 normal knees and 19 knees with confirmed pivot shift. They found that knees with a torn ACL exhibited significantly greater internal rotation than intact knees for angles of flexion less than 158. At 58 flexion, external rotation was also greater for the injured knees. This was confirmed by Markolf et al. w68x who also measured the stiffness of the rotation of these knees and found that internal rotation stiffness decreased by an average of 16% for the injured knees, while no significant changes were measured for external rotation stiffness. Shoemaker and Markolf w69x measured the total change of rotary laxity in vivo under " 10 Nm torque as 478 at 908 knee flexion. At 208 knee flexion, the tibial rotation was 338 with the hip flexed and 418 with the hip extended. This increase in rotation with the hip extended could be a contributory factor in allowing the pivot-shift motion to occur with an injured ACL. Presumably, this suggests that passive tension in the hamstrings limits tibial rotation and could also explain the common clinical situation where the pivot-shift test is conducted with the hip near to extension. 9.3. Passi¨ e coupled rotation Markolf et al. w70x measured passive coupled rotation of the tibia at different flexion angles on cadaveric knees; the joint was passively flexed or extended against gravity by clamping the femur and lifting the tibia by hand. All tibiae rotated externally with respect to the femur when extended Ž n s 35.. This work was confirmed by Kurosawa et al. w71x. This coupled rotation is known as the ‘screw-home’ mechanism. 10. Rotational axis of the pivot shift The pivot shift has been described as a sudden rotation of the tibia relative to the femur for an ACL-injured knee under a valgus torque at low angles of knee flexion w43x. This rotation is a 3-dimensional motion which occurs about a series of axes. These axes can be simplified as a single rotation about one ‘helical’ axis by taking the average during the pivot

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shift. This then reduces the motions to a single rotation Žmagnitude. about a single axis Ždisplacement.. This is referred to as a coupled rotation. In clinical terms it is more explanatory to describe coupled rotations in terms of rotations about the internalrexternal, abductionradduction and flexionrextension axes, rather than a single helical axis. For example, a recent cadaveric study has described the pivot shift as a sudden external rotation of the tibia of 348 over a flexion range of 88 between 20 and 658 flexion w72x. This describes the coupled rotation of the pivot shift in terms of two component rotations ŽFig. 2.. The inclination of the helical axis can be found from the ratio of the rotation change to the flexion change. The axis of tibial rotation is important during that sudden movement so that this can be correlated to the structures or geometry of the knee joint. Also, the change in flexion axis due to sectioning the ACL has been analysed by planar radiography w24x as well as in three dimensions w73x, although the exact meaning of the ‘position’ in a two dimensional analysis is doubtful, in view of the inclination of the axis. Shaw and Murray w74x conducted cadaveric experiments on 11 knees and measured the axis of rotation of the tibia with flexion angle. At each fixed angle of flexion they loaded the quadriceps and measured the tibial rotation. The axis of rotation was found to be at the medial inter-condylar tubercle of the tibial plateau for 0]308 flexion. When the ACL was transected the mean position of the axis was unaffected, but it became unstable compared to intact joints. Further dissection of the PCL had little effect. A second protocol was used whereby the tibia was rotated manually at fixed angles of flexion Ž158 and 308.. The axis of

Fig. 2. Two component rotations which give the coupled rotation known as the pivot shift.

rotation was found to be exactly the same as for the quadriceps-induced rotation Žfrom Matsumoto w43x.. Brunet et al. w75x found that the axis of tibial rotation was located approximately between the tibial spines at all flexion angles for intact and injured knees Žfrom Matsumoto w43x.. For the intact cadaveric joint Trent et al. w76x found that the tibial axis of rotation was located ‘close to the centre of the tibial plateau’ at all flexion angles, except at full extension, when it moved slightly medially. Hughston et al. w6x found the rotational axis of the pivot shift to be at the PCL tibial insertion. This has become commonly accepted and was promoted by Norwood et al. w32x. More recently, in a 3-dimensional study of the pivot shift, Matsumoto w63x clearly measured the axis of the pivot shift to be about the MCL. This corroborates the work presented earlier that the MCL must be intact for the pivot shift to occur ŽSection 8.4.. 11. The role of the iliotibial tract in the pivot shift The iliotibial tract ŽITT. plays a major role in the pivot shift. However, there is some disagreement as to the precise effect it has. Some of this confusion can be explained in the light of the two tests of the pivot shift, one of which elicits a sudden reduction, the second of which elicits a sudden subluxation. Slocum et al. w5x discussed the pivot-shift tests and described the need for the ITT to be intact when flexing the knee in the reduction test, so that when the fibres of the ITT luxate posteriorly during flexion this assists the tibia in reducing Žexternal rotation, Figs. 3 and 4., or in the subluxation test, so that as the knee is extended from the reduced position the ITT fibres luxate anteriorly and thus assist the sudden anterior subluxation of the lateral tibial plateau. This implies that the ITT must be intact for both the reduction and subluxation pivot-shift tests to occur. Others have confirmed this finding in the clinical situation w15,77,78x, although Losee w61x suggested that the ITT only needed to be intact for the reduction test, as with the subluxation test the knee could be rotated externally by hand in the flexed position, without the action of the ITT. Similarly, Wood and Dandy w79x and Matsumoto w43x found that the ITT needed to be intact for the reduction test. They did not discuss the subluxation test. Galway and MacIntosh w22x, when describing the reduction test, commented on both the necessity for a substantial subluxation of the lateral tibial plateau close to extension followed by the reduction. They found that this substantial subluxation did not occur without some injury to the ITT. The anatomical and clinical studies of Terry et al. w38,62x confirmed this

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causes both external rotation of the tibia Žin the range 10]908 flexion. and valgus rotation of the joint Žin the range 0]1208 flexion. when loaded. Clearly, the position of the ITT may affect the pivot shift, but there is still disagreement in this area. One of the causes of the confusion is that some of the literature focuses on the translations and rotations involved in the pivot shift, whereas others stress the suddenness of the motions. It is clear that a knee with a very lax lateral compartment can be forced to mimic the translations and rotations of the pivot shift. However, in these cases the sudden reduction or subluxation is not reproduced. The full effect of the ITT may be explained by an analysis of the articular geometry related to the ITT geometry combined with an assessment of the suddenness of the motion. Fig. 3. Reduction of the anteriorly subluxed lateral tibial plateau due to the posterior luxation of the ITT fibres. There are two positions of stability: Ža. the subluxed position; and Žb. the reduced position; these positions are related to the peak of the convexity of the lateral tibial plateau.

finding. Bach et al. w16x also conducted the reduction test and found that the pivot shift was easiest to elicit with the hip in abduction ŽSec. 6.. This will relax the ITT and thus these experiments suggest that the ITT acts to diminish the severity of the pivot shift. They also conducted some preliminary cadaver dissections and found that cutting the ITT eliminated the effect of hip position on the pivot-shift phenomenon. In contrast with the studies above, the cutting study of Fetto and Marshall w42x showed that sectioning of the ITT increased the severity of the pivot shift, however, they did not distinguish between the reduction and subluxation tests. Recently, Kwak et al. w80x found that the ITT has a small effect on normal knee kinematics in vitro. It

12. Geometrical effects on the pivot shift

There has been some discussion on the effects of the geometry of the femoral condyles and tibial plateaux on the pivot shift. For example, Matsumoto w43x said that the pivot shift occurs as a result of a complex interaction between the geometry of the lateral articulating surfaces and the soft tissues. Matsumoto further discussed this in 1990 w63x; he proposed that, because of the geometry of the condyles, the anteriorly subluxed tibial plateau relocates suddenly when a reducing force applied by the ITT becomes greater than the resistance exerted. Therefore it seems that the convexity of the lateral tibial plateau allows two positions of stability } the subluxed and reduced positions } depending on how the joint load relates to the peak of the convexity ŽFig. 3..

Fig. 4. The position of the tibial plateau during the pivot-shift motion.

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13. Treatment of the pivot-shift instability Treatment of the pivot shift has focused on observing the symptoms of instability Žrotation and translation. and treating the structures which have been injured Žthe ACL or lateral structures .. The efficacy of the procedures outlined below will be a result of the initial functional result combined with the longterm effect. Conservative treatment of the pivot shift has been tried by many clinicians w27,81,82x: Dahlstedt and Dalen ´ w83x reviewed the results from approximately 20 such articles. They concluded that poor results predominate in unselected groups of patients, while selected patients tended to be those who displayed only a minor pivot shift and who could modify their activity accordingly. The primary treatment is to reconstruct surgically. A number of surgical procedures have been developed to treat the pivot shift. Initially these focused on primary repair, however, the results presented have not been satisfactory in all cases. Extra-articular reconstruction has been very popular and has given some good results. More recently surgical procedures have focused on intra-articular reconstructions to specifically address the injury to the ACL and not just focus on the motion of the lateral compartment. Various combinations of all procedures have been tried. Comparison between different procedures is difficult, because the waiting time to operation or clinical follow-up periods are not the same, the assessment techniques vary and there is also inter-surgeon variation. Inter-patient variation is also a contributory factor as is the type of activity in which the patients partake. It must be noted that not all the procedures outlined below addressed the pivot-shift instability of the knee specifically. In addition, they addressed the anterior laxity of the knee. However, in all the cases reviewed from the literature the pivotshift test was used as a pre- and post-operative clinical assessment. 13.1. Bracing the pi¨ oting knee Knee braces serve to limit some motion of the joint by fixing a mechanical device to the skin externally. Through rigid struts, hinges and elasticated strapping different knee braces focus on limiting specific motions, such as the internalrexternal rotation of the tibia or the anterior displacement of the tibia. Nicholas w84x advocated the use of a specific knee brace and stated that the ‘pivot shift can be reduced in time; with control of laxity, the knee can be used normally’. Mishra et al. w85x compared four types of knee braces and found that on average the braces reduced the grade of the pivot shift by 0.8 grade. In no instance

did the knee brace completely eliminate the pivot-shift instability. 13.2. Primary repair and augmented primary repair Primary repair of the ACL has the specific aim of allowing the ruptured ligament to repair either using a suture technique or a type of ‘scaffold’ structure which augments the primary repair and provides a structure for the ligament to grow along. Primary repair is severely limited by the lack of fixation strength of the sutures in the ruptured ligament until the expected biological repair or remodelling is effected over time. The scaffolding structure provides a stronger initial repair. A benefit of this technique is that the orientation of the repaired ligament is expected to be comparable to that of the initial intact ligament and there is the possibility of maintaining the proprioceptive function w86x. However, the tension and stiffness of the repair may not simulate the intact conditions initially, or perhaps in the long-term. Primary repair would now be conducted only on acute ruptures of the ACL, where much of the ligament material is still present. In chronic cases, very little of the ligament fibres remain and primary repair is not feasible. Results of these procedures have been mixed w7,33,87]89x. 13.3. Extra-articular stabilisation Extra-articular stabilisation specifically addresses the motion in the lateral compartment of the symptomatic knee. This does not seek to replace the injured ACL, but to address one of the main functions of the ACL which is to stop the anterior subluxation of the lateral tibial plateau which then reduces to produce the pivot-shift motion. Typically, in these procedures some autogenous material, such as a strip of the ITT is re-routed to tighten the lateral compartment of the knee. Biomechanical analysis of extra-articular stabilisation in vitro w66x has shown that internal rotation of a reconstructed knee tightened the lateral extra-articular reconstruction, whereas external rotation did not. This demonstrates that extra-articular stabilisation can stop or reduce the anterior subluxation of the lateral tibial plateau. A drawback of such procedures is that some morbidity to the lateral structures of the knee joint is caused by harvesting structures, such as the iliotibial tract or biceps tendon. According to Losee et al. w18x, extra-articular stabilisation is superior to ACL reconstruction, because ‘the ACL had too short a lever arm to be effective in preventing rotation’. This must be seen in the context of the cadaveric work of Matsumoto and Seedhom w43,90x who found that ‘extra-articular lateral stabilisers whether they be of autogenous tissue or prosthetic

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materials, cause greater constraints on the tibia thereby reducing the amount of physiological tibial rotation occurring in a normal joint’. Various techniques for extra-articular stabilisation have been proposed and used. The ‘sling and reef’ operation of Losee et al. w18x is typical of the procedures employed. This procedure was specifically proposed to address the pivot-shift symptoms which they believed occurred due to ACL injury with some stretching of the lateral capsule. They reported that after a 1.5-year follow-up, 6% Ž n s 3r50. had positive pivot shifts. These figures are not confirmed by other studies in which 15]80% of the knees displayed a pivot shift post-operatively w51,82,91x. Summarising, it may be concluded that lateral reconstructions alone have not provided an effective reduction of the pivot-shift instability in all cases, despite the apparent mechanical efficiency of their placement. 13.4. Intra-articular ACL reconstructions Intra-articular reconstructions have used a variety of materials including autogenous grafts, allografts, xenografts, artificial ligaments, or a combination of these. Autogenous materials used for ACL reconstruction have included semitendinosus tendon, gracilis tendon, meniscus, iliotibial tract and patellar and quadriceps tendon. The objective of these intra-articular reconstructions is to reproduce the anatomy and function of the ACL through graft structure, placement, fixation, strength and tensioning. In addressing the pivot-shift motion the placement of an ACL substitute will provide a tensile restraint in the knee joint which will reduce the ability of the lateral compartment of the knee to pivot about the MCL and hence stop the pivot-shift motion from occurring. 13.4.1. Autogenous graft Currently the most widely used autogenous graft is a portion of the patellar tendon Žeither the middle one-third or the medial third.. This has been found to withstand maximum loads that are greater than those for the ACL w92x. Long-term success in reducing the pivot shift by using a central third patellar tendon of up to 100% has been presented in the literature w36,93,94x. Another technique in use is the quadriceps tendon graft which is similar to a middle third patellar tendon operation in which a patellar bone block is excised with some of the tendon tissue from the rectus femoris. The pivot shift was present in 10 to 45% of knees 5.5 years post-operatively w95,96x. Other autogenous material used for intra-articular reconstruction has been fascia lata, distal ITT and combinations of hamstring tendons. Noyes et al. w92x

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found that these structures in isolation could develop loads of only up to 70% of that of the anterior cruciate ligament. This would imply that reconstructive procedures using these structures rely on the graft strengthening with time, unless they are used in multi-strand configurations. Windsor and Insall w97x followed-up 62 patients who had had a bone-block ITT transfer intra-articular reconstruction of the ACL. The patients all had giving way Žbuckling. pre-operatively, reducing to 12% with a positive pivot-shift test 9 years post-operation. Ellera Gomez and Marczyk w98x used a loop of semitendinosus tendon and found that for an average follow-up of 3 years, none of the knees which displayed the pivot shift pre-operatively did so after. Sgaglione et al. w99x also used the pes anserinus tendons and found that for a 36-month follow-up, 5% of the acutely operated group displayed the pivot shift. This compared to 18% of the chronic group. More recently the combination of two hamstrings grafts Žgracilis and semitendinosus. has been used in a doubled configuration, giving a four bundled graft. This has been shown to be as effective as the patellar tendon graft w100,101x. 13.4.2. Allograft Biomechanically an allograft is similar to an autograft except for two important conditions: 1. there is less morbidity at the site of reconstruction for the patient; and 2. due to the threat of disease, allografts are often irradiated and thus the mechanical properties of the graft are affected. Comparison with autogenous graft is difficult, because less data is available from the literature. Indelicato w102x compared freeze-dried and freshfrozen patellar tendon allografts after 2]3 years of which 11 out of 41 had some form of pivot shift present Ž‘trace’ pivots were included.. They also conducted another study using fresh frozen patellar tendon allografts w103x. At 27 months follow-up, 22% of the knees displayed the pivot shift. Noyes and Barber-Westin w104x conducted a retrospective long-term study on the results of reconstruction of the ACL with human allograft Žusing either the patellar tendon or the fascia-lata .. Pre-operatively 100% had at least a grade 2 pivot shift. At 5]9 years follow-up, 97% had Grade 0 or 1 and 3% had Grade 2 pivot shifts. However, once again the pivot shift was not the only indicator for a failed ligament } in some instances the KT-1000 data indicated excess anterior laxity with no pivot shift present, but this was still deemed a failure. A recent cadaveric study has confirmed the finding that a knee can have excess ante-

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rior laxity due to isolated ACL injury and yet not produce a pivot shift w105x. 13.4.3. Prosthetic Prosthetic implants have had a chequered history. This can be attributed to graft placement as well as bio-incompatibility of the graft. The benefits of prosthetic implants are that they can provide a strong graft without any morbidity due to the harvesting of autogenous grafts, or risk of disease transmission associated with allografts. The size can also be tailored to suit the patient. Much attention has focused on the Gore-Tex ligament. Results have been mixed, with a high incidence of sterile effusion. Up to 40% of knees have been shown to pivot after short follow-up periods w106,107x. Another well-known prosthesis, the Dacron ŽStryker. ligament was widely used, initially as a ligament augmentation, but more recently it has been proposed for ‘salvage’ cases. Lukianov et al. w108x used the Dacron ligament in 80 salvage cases and at 24 months post-operatively 24% had positive pivot shifts. Fifty-nine of these implants had autogenous augmentation, but their results showed no difference between the two procedures. The Leeds]Keio prosthetic ligament w109x is described as a scaffold-type device which can give a new structure with the appearance of a normal ACL w110x. However, results have shown that pivoting is present in up to 50% of the knees w59,111x. Similar values have been shown for the Apex implant w112,113x. 13.4.4. Augmentation As for the primary repair of the ACL, intra-articular reconstruction is sometimes augmented. The most common augmentation device is the Kennedy Ligament Augmentation Device ŽLAD.. This serves to strengthen the reconstruction and provide a ‘scaffold’ for the ingrowth of natural collagen material. However, follow-up studies have shown that this device has no significant additional effect in halting the pivot shift w114,115x.

articular procedures, in addition to intra-articular repair, have a role in controlling antero]lateral rotatory instability of the knee. They justified this from their 8.5-year follow-up of 14 patients in whom a patellar tendon ACL reconstruction was augmented with a MacIntosh lateral tenodesis: they showed that the presence of the pivot shift did not increase with time, despite a progressive increase in anterior laxity. Kohn and Wirth w118x presented their rehabilitation procedure following reconstruction of the cruciates. They promoted the addition of an extra-articular ITT rerouting to their intra-articular reconstruction to address the pivot-shift motion specifically. They did not present any results of this procedure. This conclusion is confirmation of the earlier excellent clinical work by Kochan et al. w119x in which, 3 years post-operative, EA reconstructions were compared with those in which the EA reconstruction was combined with a patellar tendon IA reconstruction. They found that the knees with EA reconstructions alone were significantly more lax at 208 knee flexion than those combined with the IA reconstruction, but that the ‘elimination of the pivot shift was comparable in both groups’. Again, there is a plethora of results from the literature with little biomechanical analysis. Amis and Scammell w66x conducted a cadaveric study and found that for isolated ACL deficiency ‘there is no biomechanical basis for using an EA reconstruction alone and no significant biomechanical advantage from adding an EA reconstruction to an IA reconstruction’. This is confirmed by some clinical data w120,121x. They did not look at the pivot-shift motion in this study, but focused on ‘static’ laxity tests in which fixed loads or torques were applied and anterior]posterior drawer and internalrexternal rotation of the tibia were measured. It was noted that the EA reconstruction could have a role if the peripheral tissues were damaged or stretched. Other clinical data have shown some good results with 0]43% pivoting w47,98,122]125x. 13.6. Summary of treatments of the pi¨ ot shift

13.5. Combination of intra- and extra-articular reconstruction The addition of extra-articular procedures ŽEA. to intra-articular ŽIA. reconstruction is promoted by some researchers to diminish deleterious forces and tibial displacements w116x. Also, it has been mentioned that in some instances where the pivot shift is present the lateral structures are also injured or stretched ŽSec. 8.2.. The extra-articular lateral augmentation will address the lateral injuries and the intra-articular reconstruction will address the ACL injury. Zavras et al. w117x suggested that lateral extra-

Bracing is used in reducing the symptoms of the pivot shift, but the results are not as encouraging as repair or reconstruction of the ACL. Primary repair is rarely recommended other than in the acute cases. Intra-articular reconstructions have seen good results in the medium term. Extra-articular reconstructions on their own have sometimes provided good results, however, in general they are not as effective in eliminating the pivot shift over a longer period of time. Intra-articular reconstructions fix the graft ] either autogenous, allograft or prosthetic } at the anatomical ACL insertions, thus attempting to reproduce the

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natural force distributions within the joint. Extraarticular reconstructions address the anterior displacement of the lateral tibial plateau and may have a role in chronic reconstructions, after stretching-out of the lateral structures. 14. Kinematic studies Early on it was found that the kinematics of the knee joint in different planes deviated significantly from the normal for those patients with an ACL injury w24x. This has led to much work done in vivo and in vitro on the kinematics of the pivot shift and of normal motion of those knees which display the instability. Although the free-body kinematics of the knee joint is, of course, a 3-dimensional problem, previous workers have tended to treat it as though it were sagittal plane motion in two dimensions. 14.1. In ¨ i¨ o kinematics Tamea and Henning w27x assessed the instant centre of rotation in the sagittal plane for eight patients with either partial or complete tears of the ACL Žassessed by arthrography or arthroscopy. over a flexion range of 0]908. The knees that displayed the pivot shift showed marked deviation from the normal pattern of instant centre of rotation. They concluded that abnormal compression forces in the posterior aspect of the joint resulted and thus localised articular cartilage destruction was expected. In the same way Gerber and Matter w33x also conducted instant centre analysis. They found that the instant-centre pathway in knees with an acutely ruptured ACL had a specific abnormality. This ‘corresponded with a positive pivot-shift sign, but was present even when that sign was not clinically detectable’. The loading applied to the joints whilst conducting the instant-centre analysis was not described in their article and we must, therefore, question whether they were actually measuring the changes in instant centre during the pivot-shift motion or whether they were just correlating the pivot-shift motion with a characteristic planar motion. This is not clear from their article. Peterson et al. w126x found four patients with ACL deficiencies who could voluntarily produce the pivotshift phenomenon by sitting in a chair and flexing the knee between 808 and 908 with the foot flat on the floor in neutral or slight external rotation. In all the cases the pivot-shift manoeuvre could be produced by electrical stimulation of the popliteus muscle only. There are two important things to be noted from this work: 1. the pivot shift that was elicited was not at the low angles of flexion described in the literature; and

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2. muscles } and the popliteus muscle in particular } play an active role in stabilising the knee joint, but they can also be significant contributors to instabilities of the knee. Karrholm et al. w127x measured the 3-dimensional ¨ kinematics of 20 patients with a previous tear of the ACL. They analysed the simultaneously occurring translations of the tibial plateau which showed abnormal displacements of both the medial and the lateral tibial condyles. However, they proposed that the primary instability resulting from ACL tears was an anterior instability and they said ‘the rotary component of the instability is small’. Allum et al. w128x used a triaxial electrogoniometer to measure the pivot-shift motion in vivo. They found that although the pivot-shift motion was visible to the examiners, in many instances the angular results were not indicative. They ascribed this to the fact that the pivot shift is a complex 3-dimensional motion with both rotations and translation Žin this case the translations were not measured., however, their results might be more indicative of the lack of accuracy of skin-mounted instruments. In contrast, Gillquist and Messner w129x found a significant difference in the 3-dimensional kinematics of the pivoting compared to non-pivoting, however, they only described this difference in terms of anterior translation of the tibial plateau. They did not focus on the anterior translation of the lateral tibial plateau or reduce the data to give rotations of the tibia about its long axis. 14.2. In ¨ itro kinematics Lipke et al. w39x measured the kinematics of 17 cadaver knees in an Oxford-type of rig. Their results pointed to ACL deficiency causing two coupled effects: 1. increased internal rotation of the tibia; and 2. the centre of rotation about the long axis of the tibia shifted into the medial compartment. ACL insufficiency must be present for antero]lateral rotational instability to occur. Matsumoto w43x measured full 3-dimensional kinematics on 28 knees applying varying loads and varying injuries. He applied undefined loads on Gerdy’s tubercle to simulate ITT loading. For those knees with ACL injuries Ž n s 20. only seven actually pivoted. This work was actually a series of quasi-static tests rather than dynamic conditions. Reuben et al. w26x conducted a 3-dimensional analysis of 15 fresh human joints using a dynamic quadriceps loading pattern. Other loads were applied, such as varusrvalgus moment and internalrexternal rota-

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tory moment on the femur. They found that the pivot shift was present in all knees with an isolated ACL insufficiency. An internal femoral moment reduced the lateral compartment subluxation and increased the medial compartment subluxation and vice versa with an external femoral moment. Their conclusions were that the pivot shift arose from an isolated ACL insufficiency and the pivot shift could be elicited independently of an applied moment under simple quadriceps loading during flexion. They found little inter-specimen variation. No loads were applied on the Gerdy’s tubercle to simulate ITT loading, which contrasts with the findings of Matsumoto, who felt that ITT tension was essential to cause a pivot shift. Lane et al. w64x measured changes in rotation due to isolated ACL sectioning in 14 human cadaveric knees. They found no changes in tibial internal ]external rotation in their tests, however, all of the knees displayed the ‘clinical’ pivot for which the loading and motions were undefined. This shows that the loading applied in the controlled tests did not mimic the loading applied in the manual Ž‘clinical’. tests. Shoemaker et al. w130x measured the change in tibial rotation using an Oxford Rig for intact knees, ACL injured knees and knees reconstructed using a patellar tendon. They also measured the change in rotation for all these states when loads were applied to simulate the quadriceps. In both cases cutting the ACL did not seem to affect the tibial rotation substantially. However, in the case without quadriceps loading, reconstructing the ACL significantly reduced the tibial internal rotation for all angles of flexion. 15. Conclusions Certain fundamental questions are still unanswered pertaining to the cause of the pivot shift, the 3-dimensional kinematics and the specific procedures required to reconstruct a knee that demonstrates the pivot shift. Specifically, the role of the geometry of the knee has been much discussed, but no clear conclusions have been reached from these discussions. In the literature, Matsumoto w43x discussed the mechanism of the pivot shift in more detail, based on a series of cadaveric experiments. However, his discussion on geometry was not based on his own experiments, but was proposed as an untested hypothesis. Also, the 3-dimensional kinematics of the pivot shift, although it has been measured, notably by Matsumoto w43x and Noyes et al. w17x, has never been measured dynamically with the loads on the knee joint fully defined. The comparison between in vitro testing and the clinical situation is a difficult one to make, because of the variation in loading in the clinical situation due to the surrounding soft tissues

and the examiner’s own applied loads. Also, most of the cadaveric testing has been conducted on old specimens, whereas most of the ligament reconstructions in the clinical situation were carried out on young adults. There is still debate as to the specific cause of the pivot shift. It is unlikely that any research will be able to refute or reconcile some of the conflicting statements published in the literature, because of the notable differences in geometry, anatomy and soft-tissue properties between different knees. However, there is still much scope for clarifying and explaining these conflicting results. Many procedures have been employed to reconstruct a knee that displays the pivot shift, but these have had mixed results. A clearer understanding of the biomechanics of the pivot shift and the biomechanics of the clinical tests to demonstrate the pivot shift Žthe pivot shift; the jerk test. will provide the biomechanical engineer and surgeons with some objective information on which to base work aiming to improve the clinical results of surgical procedures designed to address this instability. Acknowledgements This work was supported by the Arthritis and Rheumatism Council. References w1x Noyes FR, Grood ES, Butler DL, Paulos LE. Clinical biomechanics of the knee } ligament restraints and functional stability. In: Mosby CV, editor. AAOS symposium on athlete’s knee. St Louis, 1980:1]35. w2x Scott WN, McMahon MS, Craig SM, Insall JN. The knee. In: Harris N, Birch R, editors. Clinical orthopaedics. Oxford: Blackwell Science, 1995:864]900. w3x Noesberger B. Diagnosis of acute tears of the anterior cruciate ligament, and the clinical features of chronic anterior instability. In: Jakob RP, Staubli H, editors. The knee ¨ and the cruciate ligaments. Berlin: Springer-Verlag, 1990:143]156. w4x Galway HR, Beaupre A, MacIntosh DL. Pivot shift: a clinical sign of symptomatic anterior cruciate deficiency. J Bone Jt Surg 1972;54-B:763]764. w5x Slocum DB, James SL, Larson RL, Singer KM. Clinical test for anterolateral rotary instability of the knee. Clin Orthop 1976;118:63]69. w6x Hughston JC, Andrews JR, Cross MJ, Moschi A. Classification of knee ligament instabilities. Part I. The medial compartment and cruciate ligaments. J Bone Jt Surg 1976;58A:159]172. w7x Kaplan N, Wickiewicz TL, Warren RF. Primary surgical treatment of anterior cruciate ligament ruptures. A longterm follow-up study. Am J Sports Med 1990;18:354]358. w8x Katz JW, Fingeroth RJ. The diagnostic accuracy of ruptures of the anterior cruciate ligament comparing the Lachman test, the anterior drawer sign, and the pivot shift test in acute and chronic knee injuries. Am J Sports Med 1986;14:88]91.

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